1. Field of the Invention
This invention relates to a real image mode finder optical system used in a lens shutter camera or the like.
2. Description of Related Art
Finder optical systems have been known in which a display, for example, a liquid crystal display, is placed to convey finder information such as that on a field frame. In recent years, cameras, each having the function of switching the field frame of a finder optical system, for example, to that for panoramic photography, have often been used.
In conventional finder optical systems having such field frame switching means, the field frame has been switched in such a way that it is inserted or removed in a vertical or lateral direction. With this technique, however, when the field frame is switched, it is necessary to provide space sufficient for shifting this moving field frame to a nearby place. In a real image mode finder optical system in particular, a prism for erecting an image is often placed close to an intermediate image plane. Since this prism must be enlarged to erect the image, space sufficient for placing the prism is required. Thus, there is a need to make an arrangement such that the space and driving means for shifting the moving field frame do not interfere with the prism for erecting the image. This constitutes a disadvantageous factor with respect to the requirements of current times for compact cameras.
Thus, one means for solving this problem, as set forth, for example, in Japanese Patent Preliminary Publication No. Hei 8-122857 or Hei 7-270861, is proposed in which, for example, a liquid crystal display is adopted as the field frame switching means to thereby save space for the arrangement of an optical system.
Another means refers to a case where two kinds of cameras, one including a field frame switching means and the other excluding the field frame switching means by simplifying the specification of a finder optical system, are provided to supply a user with either of them according to his need. In general, it is desired that, apart from finders, camera bodies have structures in common with one another. For this purpose, a finder optical system is proposed which is capable of having two kinds of specifications in such a way that one of them includes the switching mechanism of the field frame and the other excludes the switching mechanism. This finder optical system will be described in detail below.
In a finder optical system of a type in which a photographic field is changed by moving the field frame, the arrangement of the optical system in a case where the switching means is included is exactly the same as that in a case where the switching means is excluded, with the exception that the moving field frame is placed or not. However, in a finder optical system in which the photographic filed is changed, for example, by a liquid crystal display, if the display is removed to simplify the specification of the finder optical system, an optical path length between an objective system and an ocular system will be changed, and thus an image will cease to be normally formed, with a resulting shift in diopter.
Thus, in Hei 7-270861 mentioned above, the liquid crystal display is used in a case where the field frame switching means is required, while in a case where the liquid crystal display is not placed, a glass or plastic plane-parallel plate is used instead thereof. In this way, the same optical path length is provided in both cases so that a difference in diopter does not arise between them.
In this finder optical system, however, the liquid crystal display or the glass plate located at a position equivalent thereto is placed at the focus position of the objective system. Hence, if dirt particles adhere to the display which is brought to a focus, they become liable to appear to the eye when an observer looks through the finder. Moreover, the use of a liquid crystal for the display easily brings about circumstances such that dirt or dust particles are liable to adhere to the display because of its static electricity, and thus great care must be exercised in assembling the finder.
In recent years, since there is a tendency to decrease the entire finder in size for compactness, it is desired to reduce the focal length of an ocular system (a Lupe system). Consequently, the magnification of the Lupe system is improved, and the problem arises that adhering dirt particles and uneven edges of the field frame (a state of rough edges by working) becomes conspicuous.